Distillation method and apparatus

ABSTRACT

A distillation method and apparatus having application to the distillation of air in which liquid production make is varied by varying the pressure ratio across a turboexpander used in generating refrigeration. The pressure ratio is varied by varying the pressure of a compressed stream fed to the turboexpander. This is done by solely compressing such compressed stream by a first booster compressor during a low rate of production of liquid products. During a high rate of production of liquid products, the compressed stream is also compressed within a second booster compressor. The second booster compressor is driven by a variable speed drive to allow a variety of liquid production rates between the low level of liquid production and the high level of liquid production.

FIELD OF THE INVENTION

The present invention relates to a distillation method and apparatus in which an oxygen and nitrogen containing feed is rectified by a cryogenic rectification process to produce at least liquid oxygen and/or liquid nitrogen products in addition to gaseous oxygen and/or gaseous nitrogen products. More particularly, the present invention relates to such a method and apparatus in which a variable speed booster compressor is utilized to vary production of the liquid products between a high rate and a low rate of liquid production.

BACKGROUND OF THE INVENTION

Oxygen and nitrogen containing mixtures are distilled within one or more distillation columns of an air separation plant. In such a plant, the feed stream is typically compressed and then cooled within a main heat exchanger to a temperature suitable for its distillation. The resultant cooled stream is then separated into its component parts through rectification within a distillation column system.

In a distillation column system used to produce oxygen and nitrogen products, a column operating at a higher pressure is thermally linked to a column operating at a lower pressure. In such a system, the compressed and cooled feed stream is rectified within the higher pressure distillation column to produce a crude liquid oxygen column bottoms and a nitrogen-rich vapor as overhead in such column. The lower pressure column can be thermally linked to the higher pressure column by a condenser reboiler that is typically located in the base of such higher pressure column. Oxygen-rich liquid that collects as column bottoms in the lower pressure column is vaporized against a condensing nitrogen-rich vapor produced in the column operating at a higher pressure or the higher pressure column. The resulting nitrogen-rich liquid can be used to reflux both the higher pressure column and the lower pressure column. Part of the nitrogen-rich liquid can be taken as a liquid nitrogen product. Part of the residual oxygen-rich liquid collected in the base of the lower pressure column can also be taken as a liquid oxygen product.

In cases in which it is necessary to obtain the gaseous oxygen product at high pressure, a portion of the feed air can be compressed and then utilized to vaporize a stream of the oxygen-rich liquid within a main heat exchanger used in cooling the feed air.

Where only nitrogen products are desired, a single column, known as a nitrogen generator, is used and the resulting nitrogen-rich liquid is condensed within a heat exchanger against vaporizing crude liquid oxygen from the base of such a column.

In any air separation plant, extra refrigeration must be generated to compensate for heat leakage and warm end losses in the main heat exchanger. This refrigeration is typically generated by compressing part of the feed air in a booster compressor and introducing the resultant compressed stream into the main heat exchanger. The stream is extracted in a partially cooled state and then expanded within a turboexpander to generate an even colder exhaust stream that is then fed into the higher pressure distillation column or the lower pressure distillation column or a single column in case of a nitrogen generator. The booster compressor can be directly coupled to the turbine.

As is well known in the art, the more refrigeration that is generated by the turboexpander, the more liquids are able to be produced and taken as products. However, as is also known, this consumes more electrical power in driving the booster compressor that feeds compressed air to the turboexpander. As such, the liquid products represent value added products for an air separation plant.

Very often, the cost of electricity that is used in powering the compression equipment within an air separation plant will vary during the time of day that the power is consumed (or some other variation of time, such as weekends, time of year, etc.). Consequently, it becomes more cost effective to make liquid products during certain hours of the day. Moreover, the demand for liquid products is very often variable and as such, a constant product slate of liquid and gaseous products from an air separation plant is not desired. In order to solve this problem, U.S. Pat. No. 5,901,579 discloses an air separation plant with a double column that is designed to produce both a liquid nitrogen product and a liquid oxygen product as well as a gaseous oxygen product at pressure. The gaseous oxygen product is produced by pumping the oxygen residual liquid within the lower pressure column and then vaporizing it within the main heat exchanger. In this plant, the product boiler compressor, the turbine, the booster compressor and the turbine are all linked by a common gear case that is powered by an electric motor. When liquid production is to be increased nominally, more air is compressed in the base load compressor that sends compressed feed air to both the product boiler compressor and the booster compressor. At the same time, the inlet guide vanes to the booster compressor are opened to permit more of the compressed feed to be compressed and thereby increase the pressure ratio and flow across the turbine. As a result, more refrigeration is generated and more liquids are able to be produced. Since the pressure ratio across the turbine has increased, more power may also be supplied to the motor powering the gear case to also increase the power supplied to the turbine booster compressor.

As can be appreciated, an air separation plant such as described above cannot be designed to efficiently operate at both a high rate of liquid production and a low rate of liquid production given the fact that rotating compression and turbine equipment on a single gear case have narrow operation ranges at which they can efficiently operate. As will be discussed, the present invention, among other advantages, provides an air separation method and apparatus in which liquid production can be varied between a low rate of production and a high rate of production and rates of production that lie between the high and low rates of liquid production.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a distillation method. In accordance with this aspect of the present invention, a first compressed stream and a second compressed stream are formed that contain oxygen and nitrogen. The first compressed stream is fully cooled within a main heat exchanger. Refrigeration is generated by further compressing the second compressed stream, partially cooling the second compressed stream within the main heat exchanger and expanding the second compressed stream, after having been partially cooled, in a turboexpander to produce an exhaust stream.

The first compressed stream and the exhaust stream are introduced into at least one distillation column configured to separate the nitrogen from the oxygen and to produce at least one liquid product stream enriched in one of the nitrogen and the oxygen. During a low rate of production of the at least one liquid product stream, the second compressed stream is formed by compressing part of a compressed feed stream within a first booster compressor coupled to the turboexpander. During a high rate of production of the at least one liquid product stream, the second compressed stream is formed by coupling a second booster compressor to the first booster compressor so that the part of the compressed feed stream is compressed to a pressure higher than that obtainable by the first booster compressor, thereby to increase the expansion ratio across the turboexpander and the refrigeration generated by the turboexpander.

The second booster compressor is driven with a variable speed drive and the speed of the variable speed drive can be varied to vary the pressure of the part of the feed stream and therefore production of the at least one liquid product stream during the high rate of production.

The feed stream can be composed of air and the feed stream can be compressed in a base load compressor to produce a compressed feed stream. The compressed feed stream is purified of higher boiling contaminants comprising carbon dioxide, water vapor and hydrocarbons within a purification unit to form a purified compressed feed stream. The second compressed stream is formed from part of the purified compressed feed stream. As such, the rate of liquid production is able to be varied between the high rate of liquid production and the low rate of liquid production.

The at least one distillation column can comprise a double column unit having a higher pressure column in a heat transfer relationship with the lower pressure column such that a nitrogen-rich column overhead of the higher pressure column is condensed against boiling an oxygen-rich liquid of the lower pressure column. The higher pressure column and the lower pressure column are connected such that a stream of a crude liquid oxygen column bottoms of a higher pressure column is expanded and introduced into the lower pressure column. Streams of nitrogen-rich liquid produced from the condensation of the nitrogen-rich column overhead, at least in part reflux both the high pressure column and the lower pressure column. The at least one liquid product stream can be at least one of a liquid product stream composed of the oxygen-rich liquid column bottoms and a liquid nitrogen product stream composed of part of one of the streams of the nitrogen-rich liquid used in refluxing the lower pressure column.

A stream of the oxygen-rich liquid column bottoms can be pumped to form a pumped liquid oxygen stream. In such case, part of the pumped liquid oxygen stream can form the liquid oxygen product stream. A remaining part of the pumped liquid oxygen stream is then vaporized within a main heat exchanger. The first compressed stream can be formed in such case by further compressing a remaining part of the compressed feed stream. The first compressed stream is then introduced into the main heat exchanger to vaporize the remaining part of the pumped liquid oxygen stream.

The one of the streams of the nitrogen-rich liquid can in any case be subcooled. However after having been subcooled, the part of the one of the streams of the nitrogen-rich liquid can form the liquid nitrogen product stream and the remaining part thereof can be introduced into the lower pressure column as reflux. A waste nitrogen stream and a gaseous nitrogen product stream can be withdrawn from the lower pressure column. The waste nitrogen stream and the gaseous nitrogen product stream can be passed through an indirect heat exchanger with the one of the streams of the nitrogen-rich liquid, thereby to subcool the one of the streams of the nitrogen-rich liquid. The waste nitrogen stream and the gaseous nitrogen product stream can thereupon be introduced into the main heat exchanger where they fully warm.

In another aspect, the present invention provides a distillation apparatus. In accordance with this aspect of the present invention at least one compressor is provided to compress a feed stream containing oxygen and nitrogen and producing, at least in part, a first compressed stream and a second compressed stream. A main heat exchanger is provided in flow communication with the at least one compressor and configured such that the first compressed stream fully cools within the main heat exchanger.

A turbine booster compression system interposed between the main heat exchanger and the at least one compressor such that the second compressed stream is further compressed and introduced into the main heat exchanger. The main heat exchanger is also configured such that the second compressed stream, after having further been compressed, partially cools within the main heat exchanger. A turboexpander is connected to the main heat exchanger such that the second compressed stream is expanded in the turboexpander to produce an exhaust stream, thereby to generate refrigeration.

At least one distillation column is connected to the main heat exchanger to receive the first compressed stream and the exhaust stream and is configured to separate the nitrogen from the oxygen and to produce at least one liquid product stream enriched in one of the nitrogen and the oxygen.

The turbine booster compression system has a first booster compressor coupled to the turboexpander, a second booster compressor and a flow control network having valves operable to be set into positions such that at a low rate of production of the at least one liquid product stream, part of the compressed feed stream is compressed within the first booster compressor, thereby to further compress the second compressed stream. During a high rate of production of the at least one product stream, the second booster compressor is coupled to the first booster compressor so that the part of the compressed feed stream is compressed to a pressure higher than that obtainable by the first booster compressor, thereby to further compress the second compressed stream and to increase the expansion ratio across the turboexpander and the refrigeration generated by the turboexpander.

A variable speed drive drives the second booster compressor such that varying the speed of the variable speed drive varies the pressure of the second compressed stream and therefore production of the at least one liquid product stream during the high rate of production.

The at least one compressor can comprise a base load compressor to compress a feed stream composed of air, thereby to produce a compressed feed stream. A purification unit in such case is connected to the base load compressor. The purification unit is configured to purify the compressed feed stream of higher boiling contaminants comprising carbon dioxide, water vapor and hydrocarbons.

The at least one distillation column can comprise a double column unit having a higher pressure column in a heat transfer relationship with the lower pressure column. As a result, a nitrogen-rich column overhead of the higher pressure column is condensed against boiling an oxygen-rich liquid of the lower pressure column.

The higher pressure column and the lower pressure column are connected such that a stream of a crude liquid oxygen column bottoms of the higher pressure column is expanded and introduced into the lower pressure column. Streams of nitrogen-rich liquid produced from the condensation of the nitrogen-rich column overhead, at least in part, reflux both the higher pressure column and the lower pressure column. The at least one liquid product stream is at least one of a liquid oxygen product stream composed of the oxygen-rich liquid column bottoms and a liquid nitrogen product stream composed of part of one of the streams of the nitrogen-rich liquid that is used in refluxing the lower pressure column.

A pump can be connected to the lower pressure column to pump a stream of the oxygen-rich liquid column bottoms. This forms a pumped liquid oxygen stream. In such case, the main heat exchanger is connected to the pump so that part of the pumped liquid oxygen stream forms the liquid oxygen product stream and a remaining part of the pumped liquid oxygen stream is vaporized within the main heat exchanger. A product boiler compressor can be interposed between the main heat exchanger and the base load compressor to further compress a remaining part of the compressed feed stream and thereby form the first compressed stream. The first compressed stream is sufficiently compressed by the product boiler compressor to vaporize the remaining part of the pumped liquid oxygen stream within the main heat exchanger.

A subcooling unit is positioned to subcool the one of the streams of the nitrogen-rich liquid prior to being introduced into the lower pressure column as reflux. The subcooling unit is connected to the lower pressure column such that after having been subcooled, the part of the one of the streams of the nitrogen-rich liquid forms the liquid nitrogen product stream and a remaining part thereof is introduced into the lower pressure column as the reflux. The subcooling unit is also connected to the lower pressure column such that a waste nitrogen stream and a gaseous nitrogen product stream pass an indirect heat exchange with the one of the streams of the nitrogen-rich liquid. The main heat exchanger is in turn connected to the subcooling unit and is configured such that the waste nitrogen stream and the gaseous nitrogen product stream are introduced into the main heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the following illustration in which:

FIG. 1 is a schematic process flow diagram of an air separation plant used in carrying out a method of the present invention; and

FIG. 2 is an illustration of a performance curve of a variable speed compressor utilized in connection with the present invention.

DETAILED DESCRIPTION

A feed stream 10 containing oxygen and nitrogen, for instance air, is distilled within a distillation apparatus 1 in accordance with the present invention. Distillation apparatus 1 is designed to produce gaseous and liquid oxygen products as well as gaseous nitrogen and liquid products. It is understood, however that this is for purposes of illustration only and the present invention has applicability to a distillation apparatus in which only a single column is used to produce gaseous and liquid nitrogen products such as in a nitrogen generator as has been described above. Additionally, although not illustrated, a distillation apparatus and method in accordance with the present invention might also include other columns that, for example, are specifically designed for argon and rare gas production.

Feed stream 10 is compressed within a base load compressor 12 that may be an intercooled, integral gear compressor with condensate removal. The resultant compressed feed stream 14, if air, is then purified within a prepurification unit 16. Prepurification unit 16 is well known in the art typically contains beds of alumina and/or molecular sieve operated in accordance with the temperature and/or pressure swing adsorption cycle in which higher boiling impurities are adsorbed. As known in the art, such higher boiling impurities are typically, carbon dioxide, water vapor and hydrocarbons. While one bed is operating, another bed is regenerated. Other processes could be used such as direct contact water cooling, refrigeration based chilling, direct contact with chilled water and phase separation.

It is understood, however, that oxygen and nitrogen containing gases may be formed from bleed air from a compressor of a gas turbine. In such case, a plant might be constructed without a base load compressor or the like since the feed would be obtained at pressure. Additionally, other sources of oxygen and nitrogen might be used as a feed that do not require separate impurity removal. For example, the feed to an apparatus of the present invention might be obtained from another air separation plant.

A main heat exchanger 18 is in flow communication with the base load compressor 12. Main heat exchanger 18 is configured such that a first compressed stream 20 is fully cooled within the main heat exchanger 18 and a second compressed stream 22 is partially cooled within the main heat exchanger 18. First compressed stream 20 and second compressed stream 22 are made up of the compressed feed stream 14. In case of air, as discussed above, the compressed stream 14 is purified by prepurification unit 16.

It is understood, that main heat exchanger 18, although illustrated as a single unit, could in reality be two units in which a separate product boiler is provided for boiling liquid oxygen. Additionally, the main heat exchanger 18 could be further split into sections at the warm and cold ends thereof. As such, the term “main heat exchanger” as used herein and in the claims encompasses both a single unit and a system of heat exchangers as well known in the art. Brazed fin aluminum heat exchangers could be used. However, other possible designs known in the art could also be used. Furthermore, the term, “partially cooled” as used herein and in the claims with respect to the main heat exchanger means cooled to a temperature between the temperatures of the main heat exchanger 18 at the warm and cold ends thereof. The term, “fully cooled” as used herein and in the claims means cooled to a temperature existing at the cold end of the main heat exchanger 18. The term, “fully warmed” as used herein and in the claims, means warmed to the temperature of the main heat exchanger at the warm end thereof.

As will be discussed, apparatus 1 is designed to produce a pressurized oxygen stream that will be vaporized within the main heat exchanger. As such, first and second compressed streams 20 and 22 are formed by dividing compressed feed stream 14, after having been purified, into a first part 30 and a remaining part 32. First part 30 of compressed stream 14 is compressed by a turbine booster compression system that will be discussed hereinafter. The second part 32 of the compressed feed stream 14 is compressed and a product boiler compressor 34 that again, can be an integral gear compressor with condensate removal between stages. It is understood, that product boiler compressor 34 would be deleted in an embodiment of the present invention that did not involve the vaporization of pumped liquid oxygen. Such an embodiment might employ only a single compressor, such as base load compressor 12 or if bleed air from a gas turbine, the compressor would be the gas turbine compressor.

The turbine booster compression system includes a first booster compressor 36, second booster compressor 40 and associated drives, after-coolers and piping. During a low rate of liquid production of the liquid products, the first portion 30 of compressed feed stream 14 is compressed within a first booster compressor 36. After removal of the heat of compression by an after-cooler 38 the resultant second compressed stream 22 is introduced into main heat exchanger 18 and is discharged from an intermediate location thereof. During a high rate of liquid production, in order to increase the refrigeration and therefore, allow the production of more liquids, the first part 30 of the compressed feed stream 14 is further compressed within the second booster compressor 40 to pressure that is higher than that obtainable in the first booster compressor 36. After removal of the heat of compression within an after cooler 42, the resultant second compressed stream 22 is withdrawn from the main heat exchanger 18, again in a partially cooled state, and is introduced at a higher pressure into turboexpander 24 to produce more refrigeration.

Second booster compressor 40 is coupled to first booster compressor 36 by a flow control network 44 of the turbine booster compression system having a first control valve 46 and a second control valve 48. When control valve 46 is set in the open position, control valve 48 is set in the closed position. As such, second compressed stream 22 is formed solely by compressing the first part 30 of compressed feed stream 14 by first booster compressor 36. As stated above, this occurs during low rates of liquid production where less refrigeration is desired. During higher rates of liquid production, first control valve 46 is set in the closed position and second control valve 48 is set in the open position. The compressed stream emanating from first booster compressor 36 is able to pass through a conduit 50 in flow communication with second booster compressor 40 and then from the outlet of second booster compressor 40 through a conduit 52 to main heat exchanger 18 to form second compressed stream 22. As described above, this is done during periods in which higher refrigeration is desired and therefore a higher rate of production of the products.

Distillation column system 28 has a higher pressure column 54 and a lower pressure column 56. Higher pressure column 54 is provided with mass transfer contacting elements 58 and 60 and lower pressure column 56 is provided with mass transfer contacting elements 62, 64, 66 and 68. Higher pressure column 54 and lower pressure column 56 are so named in that higher pressure column 54 operates at a higher pressure than the lower pressure column 56. The mass transfer contacting elements 58 through 68 can preferably be structured packing or other known elements such as dump packing or sieve trays or combinations thereof. However, in both such columns, liquid and vapor phases of the mixture contained in feed stream 20 are contacted within such elements and an ascending vapor phase is produced that becomes evermore rich in the nitrogen and a liquid phase descends that becomes evermore rich in the oxygen. In the illustrated embodiment, the exhaust stream 26 is introduced into the base of higher pressure column 54 to initiate the formation of the ascending vapor phase. First compressed stream 20, being at a much higher pressure than either of the higher pressure column 54 and the lower pressure column 56, is expanded to a higher pressure column pressure by way of an expansion valve 70 and divided into a first portion 72 that is introduced into the higher pressure column 54 and rectified and second portion 74 that is expanded again in an expansion valve 76 to a pressure suitable for its introduction into lower pressure column 56 where such stream is also rectified. It is understood that liquid expanders could be used in place of the expansion valves to recover additional power.

Within higher pressure column 54, the distillation produces a crude liquid oxygen column bottoms 78. A stream 80 of the crude liquid oxygen column bottoms 78 is expanded in an expansion valve 82 or as discussed above, a liquid expander and then introduced into lower pressure column 56 for further refinement. Within lower pressure column 56, an oxygen-rich liquid 84 collects as a column bottoms. Additionally, at the top portion of the higher pressure column 54, a nitrogen-rich vapor 86 collects. Higher pressure column and lower pressure column 54 and 56 are linked via a condenser reboiler 88. A stream of the nitrogen-rich vapor 90 that has collected within higher pressure column 54 is introduced into condenser reboiler 88 to produce a nitrogen enriched liquid stream 92. One part 94 of nitrogen-rich liquid stream 92 is at least in part introduced into the top of lower pressure column 56 as reflux to initiate the formation of the descending liquid phase. Similarly, the other part 96 of nitrogen-rich liquid stream 92 is introduced into the top of higher pressure column 54 to initiate the formation of the descending liquid phase.

An oxygen-rich liquid stream 98 is withdrawn from the bottom of the lower pressure column 56 and consists of the residual oxygen-rich liquid that is not vaporized by condenser reboiler 88. This oxygen-rich liquid stream is pumped by a pump 100 to produce a pumped liquid oxygen stream 102. A part 104 of pumped liquid oxygen stream 102 forms the liquid oxygen product stream. A remaining part 106 vaporized within the main heat exchanger 18 to produce a gaseous oxygen product stream 108. The vaporization is effectuated by the remaining part 32 of compressed feed stream 14 that has been recompressed within product boiler compressor 34. As such, first compressed stream 20 will be liquefied upon such vaporization.

The one part 94 of nitrogen-rich liquid stream is passed through a subcooling unit 110 and one portion 112 thereof can also be taken as a liquid product and a remaining portion 114 can be introduced into the top of lower pressure column 68 as reflux. Subcooling is accomplished by passing a gaseous nitrogen product stream 116 and a waste nitrogen stream 118 in indirect heat exchange with the one part 94 of nitrogen-rich liquid stream 92. This produces a warmed gaseous product stream 120 and a fully warmed waste nitrogen stream 122.

As can be appreciated, an embodiment is possible in which only liquid nitrogen product stream 112 is produced or a liquid oxygen product stream 104 is produced. As illustrated, both of such products could be produced. However, when either or both of such products are produced and more liquid is required, the second booster compressor 40 will come on the line to produce second compressed stream 22 at a higher pressure to increase the expansion ratio across turboexpander 24 to generate more refrigeration and thus produce more liquids. In addition to the foregoing, the second booster compressor 40 is driven by a variable speed drive 124, for example, a variable speed motor. Thus, at the higher rate of liquid production intermediate liquid makes are possible by increasing the speed of variable speed drive 124 or by decreasing the speed of variable speed drive 124. It is to be noted that preferably, that a direct drive is employed to drive second booster compressor 40. Oil-free bearings are preferred to support the rotating assembly within second booster compressor 40 such as magnetic bearings.

In order to bring the second booster compressor 40 on line, pressure valve 154 is opened while flow valve 48 is closed, the variable speed drive 124 is first controlled so that the pressure ratio across the second booster compressor 40 is equal to about 1.0 while recirculating flow through open valve 156. Check valve 158 prevents reverse flow through second booster compressor 40 from first booster compressor 36. Ramp up of second booster compressor 40 to stable operation is accomplished by increasing flow through second booster compressor 40 as valve 48 is opened and valve 156 and valve 46 are simultaneously closed in a coordinated manner. After second booster compressor 40 has been brought up to speed, flow valve 48 and check valve 158 will be full open and valves 156, 154 and 46 will be full closed. The speed of the second booster compressor 40 may be further increased or decreased to obtain a specific liquid production.

It is to be noted that the pressure of exhaust stream 26 is relatively constant and is at the pressure of the higher pressure column 54. As a result, flow through the flow control network 44 and specifically the first booster compressor 36 and the second booster compressor 40, will be fairly constant. There can be some variations in flow and as such, the flow of the turbine booster fluid is sensed by flow meter 130. Additionally, a pressure ratio across the second booster compressor 40 is sensed by pressure transducers 132 and 134. As illustrated, pressure transducer 132 senses the outlet pressure and pressure transducer 134 senses the inlet pressure. Electrical signals referable to the flow rate and pressures are transmitted via data transmission lines 136, 138 and 140 to a programmable logic controller 142. Controller 142 computes the pressure ratio across second booster compressor 40, namely, the outlet pressure divided by inlet pressure. Although not illustrated, controller 142 also controls operation of valves 46 and 48.

With additional reference to FIG. 2, an operating curve of the specific compressor utilized for booster compressor 40 is illustrated. This curve can be programmed as a polynomial function or a look-up table within controller 142. For a given flow and pressure ratio, the optimum rotational speed is set and a control signal is sent through data transmission line 144 of a controller associated with variable speed drive 124 to set the rotational speed of second booster compressor 40. However, during startup, the data of FIG. 2 will be used at a given flow rate sensed by flow meter 130 to select a rotational speed such that the pressure ratio is unity across the second booster compressor 40. The pressure ratio computed as a result of the pressure sensed by pressure transducers 132 and 134 will be used to fine tune the rotation speed. For example, if the pressure ratio is too high, the speed will be reduced and if too low, the speed will be increased. This fine tuning will also be done during any change in rotational speed of the second booster compressor 40.

Once second booster compressor 40 is brought up to speed, as described above, liquid production may be increased to a desired level within the capability of apparatus 1. The flow rate of the liquid nitrogen product stream 112 and the liquid oxygen product stream 104 are sensed by flow meters 146 and 148, respectively, and signals referable to the flow are transmitted by data transmission lines 150 and 152 to controller 142. If additional liquid production is desired, the controller 142 will react to data in which given pressure ratios of the second booster compressor 40 are related to corresponding liquid production rates and will implement the required speed of variable speed drive 124 and therefore, second booster compressor 40 to produce a specific pressure ratio at the current flow rate as sensed by flow meter 130. This implementation will be done by the performance data of the type shown in FIG. 2. The relationship between the pressure ratio and production of the liquid product can be determined empirically for a given plant or can be calculated and confirmed through empirical observation. The resulting data can be then programmed into controller 142 as a look up table so that any liquid product make can be preselected. The flow meters 146 and 148 and their associated flow data are used to fine tune the speed of second booster compressor 40. For example, if either the production rates of liquid nitrogen product stream 112 or liquid oxygen product stream 104 are too low after system stabilization, then the rotational speed of second booster compressor 40 will be increased and vice-versa.

While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and scope of the present invention as set forth in the presently pending claims. 

1. A distillation method comprising: forming a first compressed stream and a second compressed stream containing oxygen and nitrogen, the first compressed stream and the second compressed stream containing oxygen and nitrogen; fully cooling the first compressed stream in a main heat exchanger; generating refrigeration by further compressing the second compressed stream, partially cooling the second compressed stream within the main heat exchanger and expanding the second compressed stream after having been partially cooled in a turboexpander to produce an exhaust stream; introducing the first compressed stream and the exhaust stream into at least one distillation column configured to separate the nitrogen from the oxygen and to produce at least one liquid product stream enriched in one of the nitrogen and the oxygen; during a low rate of production of the at least one liquid product stream, the second compressed stream being formed by compressing part of the compressed feed stream within a first booster compressor coupled to the turboexpander; during a high rate of production of the at least one liquid product stream, the second compressed stream being formed by coupling a second booster compressor to the first booster compressor so that the part of the compressed feed stream is compressed to a pressure higher than that obtainable by the first booster compressor, thereby to increase the expansion ratio across the turboexpander and the refrigeration generated by the turboexpander; driving the second booster compressor with a variable speed drive; and varying the speed of the variable speed drive to vary the pressure of the part of the feed stream and therefore production of the at least one liquid product stream during the high rate of production.
 2. The method of claim 1, wherein: a feed stream composed of air is compressed in a base load compressor to produce a compressed feed stream; the compressed feed stream is purified of higher boiling contaminants comprising carbon dioxide, water vapor and hydrogen carbons within a purification unit to form a purified compressed feed stream; and the second compressed stream is formed from part of the purified compressed feed stream.
 3. The method of claim 2, wherein: the at least one distillation column comprises a double column unit having a higher pressure column in a heat transfer relationship with a lower pressure column such that a nitrogen-rich column overhead of the higher pressure column is condensed against boiling an oxygen-rich liquid of the lower pressure column; the higher pressure column and the lower pressure column being connected such that a stream of a crude liquid oxygen column bottoms of the higher pressure column is expanded and introduced into the lower pressure column, streams of nitrogen-rich liquid produced from the condensation of the nitrogen-rich column overhead, at least in part, reflux both the higher pressure column and the lower pressure column; the exhaust stream is introduced into the higher pressure column; the first compressed stream is introduced into at least one of the higher pressure column and the lower pressure column; and the at least one liquid product stream is at least one of a liquid oxygen product stream composed of the oxygen-rich liquid column bottoms and a liquid nitrogen product stream composed of part of one of the streams of the nitrogen-rich liquid used in refluxing the lower pressure column.
 4. The method of claim 3, wherein: a stream of the oxygen-rich liquid column bottoms is pumped to form a pumped liquid oxygen stream; part of the pumped liquid oxygen stream forms the liquid oxygen product stream; a remaining part of the pumped liquid oxygen stream is vaporized within the main heat exchanger; the first compressed stream is formed by further compressing a remaining part of the purified compressed feed stream; and the first compressed stream is introduced into the main heat exchanger to vaporize the remaining part of the pumped liquid oxygen stream.
 5. The method of claim 3 or claim 4, wherein: the one of the streams of the nitrogen-rich liquid is subcooled; after having been subcooled, the part of the one of the streams of the nitrogen-rich liquid forms the liquid nitrogen product stream and a remaining part thereof is introduced into the lower pressure column as the reflux; a waste nitrogen stream and a gaseous nitrogen product stream are withdrawn from the lower pressure column; the waste nitrogen stream and the gaseous nitrogen product stream are passed in indirect heat exchange with the one of the streams of the nitrogen-rich liquid, thereby to subcool the one of the streams of the nitrogen-rich liquid; the waste nitrogen stream and the gaseous nitrogen product stream are introduced into the main heat exchanger; and the waste nitrogen stream and the gaseous nitrogen product stream fully warm within the main heat exchanger.
 6. A distillation apparatus comprising: at least one compressor compressing a feed stream containing oxygen and nitrogen and producing, at least in part, a first compressed stream and a second compressed stream; a main heat exchanger in flow communication with the at least one compressor and configured such that the first compressed stream fully cools within the main heat exchanger; a turbine booster compression system interposed between the main heat exchanger and the at least one compressor such that the second compressed stream is further compressed and introduced into the main heat exchanger, the main heat exchanger also configured such that the second compressed stream after having further been compressed partially cools within the main heat exchanger; a turboexpander connected to the main heat exchanger such that the second compressed stream is expanded in the turboexpander to produce an exhaust stream, thereby to generate refrigeration; at least one distillation column connected to the main heat exchanger to receive the first compressed stream and the exhaust stream in and configured to separate the nitrogen from the oxygen and to produce at least one liquid product stream enriched in one of the nitrogen and the oxygen; the turbine booster compression system having a first booster compressor coupled to the turboexpander, a second booster compressor and a flow control network having valves operable to be set into positions such that at a low rate of production of the at least one liquid product stream, part of the compressed feed stream is compressed within the first booster compressor, thereby to further compress the second compressed stream and during a high rate of production of the at least one product stream, the second booster compressor is coupled to the first booster compressor so that the part of the compressed feed stream is compressed to a pressure higher than that obtainable by the first booster compressor, thereby to further compress the second compressed stream and to increase the expansion ratio across the turboexpander and the refrigeration generated by the turboexpander; and a variable speed drive driving the second booster compressor such that varying the speed of the variable speed drive varies the pressure of the second compressed stream and therefore production of the at least one liquid product stream during the high rate of production.
 7. The distillation apparatus of claim 6, wherein: the at least one compressor comprises a base load compressor to compress a feed stream composed of air, thereby to produce a compressed feed stream; and a purification unit is connected to the base load compressor, the purification unit configured to purify the compressed feed stream of higher boiling contaminants comprising carbon dioxide, water vapor and hydrogen carbons.
 8. The distillation apparatus of claim 7, wherein: the at least one distillation column comprises a double column unit having a higher pressure column in a heat transfer relationship with a lower pressure column such that a nitrogen-rich column overhead of the higher pressure column is condensed against boiling an oxygen-rich liquid of the lower pressure column; the higher pressure column and the lower pressure column being are connected such that a stream of a crude liquid oxygen column bottoms of the higher pressure column is expanded and introduced into the lower pressure column, streams of nitrogen-rich liquid produced from the condensation of the nitrogen-rich column overhead, at least in part, reflux both the higher pressure column and the lower pressure column; the double column unit is connected to the main heat exchanger so that the exhaust stream is introduced into the higher pressure column and the first compressed stream is introduced into at least one of the higher pressure column and the lower pressure column; and the at least one liquid product stream is at least one of a liquid oxygen product stream composed of the oxygen-rich liquid column bottoms and a liquid nitrogen product stream composed of part of one of the streams of the nitrogen-rich liquid used in refluxing the lower pressure column.
 9. The distillation apparatus of method of claim 8, wherein: a pump connected to the lower pressure column to pump a stream of the oxygen-rich liquid column bottoms, thereby to form a pumped liquid oxygen stream; the main heat exchanger is connected to the pump so that part of the pumped liquid oxygen stream forms the liquid oxygen product stream and a remaining part of the pumped liquid oxygen stream is vaporized within the main heat exchanger; the at least one compressor also comprises a product boiler compressor interposed between the main heat exchanger and the base load compressor to further compress a remaining part of the compressed feed stream and thereby form the first compressed stream; and the first compressed stream is sufficiently compressed by the product boiler compressor to vaporize the remaining part of the pumped liquid oxygen stream within the main heat exchanger.
 10. The method of claim 8 or claim 9, wherein: a subcooling unit is positioned to subcool the one of the streams of the nitrogen-rich liquid prior to being introduced into the lower pressure column as the reflux; the subcooling unit is connected to the lower pressure column such that after having been subcooled, the part of the one of the streams of the nitrogen-rich liquid forms the liquid nitrogen product stream and a remaining part thereof is introduced into the lower pressure column as the reflux; the subcooling unit is also connected to the lower pressure column such that a waste nitrogen stream and a gaseous nitrogen product stream pass in indirect heat exchange with the one of the streams of the nitrogen-rich liquid; and the main heat exchanger connected to the subcooling unit and configured such that the waste nitrogen stream and the gaseous nitrogen product stream are introduced into the main heat exchanger. 